Microwave bandpass filter with higher harmonics rejection function

ABSTRACT

A rectangular waveguide bandpass filter for transmitting fundamental electromagnetic waves f0 in a fundamental mode TE101 and attenuating second harmonic waves 2f0 therein, comprising two susceptance elements spaced apart a distance of one-third waveguide wavelength in a lengthwise direction interiorly of the waveguide to form a resonant cavity for passing a frequency band including the fundamental wave f101, attenuating the second harmonic wave 2f101, and preventing resonant frequencies of modes higher than the fundamental mode TE101 from decreasing into a frequency region below the second harmonic; and one or two adjustable screws disposed between the two susceptance elements in one or both waveguide wide walls to project into the interior of the cavity at a position which is one-twelth of the one-third waveguide wavelength susceptance element spacing and which is from an adjacent narrow waveguide wall one-third of the overall distance between the two narrow waveguide walls whereby the screws are restricted to function as one or two capacitive elements only for the TE101 mode.

United States Patent Kitazume et al.

[15] 3,657,670 1 Apr. 18, 1972 [54] MICROWAVE BANDPASS FILTER WITH HIGHER HARMONICS REJECTION FUNCTION [72] Inventors: Susumu Kitazume; Osamu Kasuga, both of Tokyo, Japan [73] Assignee: Nippon Electric Company, Limited,

Tokyo, Japan [22] Filed: Feb. 9, 1970 [21] Appl. No.: 9,926

[30] Foreign Application Priority Data Feb. 14, 1969 Japan ..44/11376 [52] U.S.Cl. ..333/73W,333/83R [51] lnt.Cl...' ..H03h 7/10, H01k7/06 [58] Field of Search ..333/70, 73, 73 C, 73 W, 95-98,

[56] References Cited UNITED STATES PATENTS 3,451,014 6/1969 Brasnahan. ..333/73 W 3,164,792 l/l965 Georgler i i ..333/73 2,476,034 7/1949 Fox ..333/83 2,629,015 2/1953 Reed ..333/73 3,422,380 1/1969 Kurocla ..333/83 3,078,423 2/1963 Lewis ..333/6 3,353,123 11/1967 Met ..333/73W Primary Examiner-H. K. Saalbach Assistant Examiner-C. Baraff Attorney-Mam & J angarathis [57] ABSTRACT second harmonic; and one or two adjustable screws disposed between the two susceptance elements in one or both waveguide wide walls to project into the interior of the cavity at a position which is one-twelth of the one-third waveguide wavelength susceptance element spacing and which is from an adjacent narrow waveguide wall one-third of the overall distance between the two narrow waveguide walls whereby thescrews are restricted to function as one or two capacitive elements only for the TE mode.

11 Claims, 15 Drawing Figures PATENTEDA R 13 m2 SHEET 10F 4 l.5fo

f Frequency lfo INVENTORS Susumu Kituzume at al. BY mm A? Mama/Mu Fig. 6

ATTORNEYS PATENTEDAPR 18 m2 3. 557'. 670

SHEET 2 UF 4 INVENTORS Susumu Kitazume e BY 77Zam a" ATTOZRNEYS PATENTEDAPR 18 I972 SHEET [1F 4 INVENTORS Susumu Kltozume et al. I

ATTORNEYS MICROWAVE BANDPASS FILTER WITH HIGHER I-IARMONICS REJECTION FUNCTION This invention relates to a waveguide-type bandpass filter and, more particularly, to a filter of this type capable of rejecting higher harmonic components with the arbitrary selectivity on the fundamental wave component maintained.

In a transmitter-receiver of a microwave communication system, a travelling-wave tube is used as the power amplifier on the transmission side. A travelling-wave tube has, however, inherent nonlinearity, due to which higher harmonics are inevitably generated at the amplification stage. Such harmonic components are not only unnecessary to microwave communication but also undesirable for the system as a whole because it requires excessive power for transmission. Such undesirable components should therefore be removed. A band pass filter is usually coupled to the output end of the TWT arnplifier for this purpose. However, a conventional bandpass filter of the waveguide type is not capable of rejecting the higher harmonic components. It allows the undesirable higher n harmonics to pass therethrough together with the fundamental components. To remove the harmonics, a lowpass filter must be employed in addition to the bandpass filter.

It is therefore an object of the present invention to provide a microwave bandpass filter which is capable of rejecting the undesirable higher harmonic components to do away with any additional filter means for the harmonic rejection purpose.

According to this invention, a novel microwave bandpass filter is provided which has sufficiently high rejection characteristic against second harmonic component, which is dominant among the higher harmonic components.

This invention is based on the fact that the higher harmonic components can be substantially suppressed by rejecting the second higher harmonic component, because higher-thansecond higher harmonics are very weak and can be neglected. Since the second higher harmonics are in the region where higher transmission modes of microwaves are concentrated, the second higher harmonics can be suppressed by eliminating the higher mode components.

Now, the invention will be described in conjunction with the accompanying drawings, in which:

FIG. 1 schematically shows a conventional bandpass filter;

FIG. 2 is a waveform diagram showing the characteristics of the bandpass filter of FIG. 1;

FIGS. 3a-3f shows various modes possible in a bandpass filter of the rectangular waveguide type;

FIG. 4 shows characteristic curves of the filter to illustrate the principle of this invention;

FIG. 5 schematically shows a bandpass filter embodying this invention;

FIG. 6 shows the characteristic curve of the filter-of FIG. 5; and

FIG. 7 through 10 schematically show modifications of the embodiment in FIG. 5.

In FIG. I, which schematically shows a perspective view of the conventional bandpass filter, susceptance elements 11 and 11', each consisting of three rods disposed in perpendicular relation to the wide plane of a rectangular waveguide 10 for TE mode propagation, are disposed at an interval of one-half of the guide wavelength Ag (namely, Ag/2) to form a cavity resonator 12. A plurality of resonators 12 and 13 are disposed in series at a spacing of Ag/4. Each of the resonators 12 and 13 has tuning screw 14 for attaining the tuned state at each of the resonators l2 and 13.

This bandpass filter has the attenuation vs. frequency characteristic as seen from the curve in FIG. 2 that a bandpass filter consisting of a waveguide having a width a of the wide plane has its selectivity only in the range where the frequency is lower than the cutoff frequency fc for the TE, mode. The cutoff frequency fc is approximately equal to c/a; where 0 denotes the velocity of light. Also, it is seen from the curve that the bandpass filter has its selectivity only in the range where the frequency f is lower than fc In the region above f rnthe characteristics become indefinite, because the existence of possible higher modes disturbs the function of the filter. Generally, the second harmonic 2f twice as high as the frequency f of the passband of the bandpass filter, is higher than fc and included in the region where disturbance is caused.

FIG. 3(a) illustrates field intensity distributions (in absolute value) of the fundamental (or dominant) mode TE which is generally used as the desirable frequency of the passband of the filter. FIGS. 3(b) through 3(f) illustrate higher modes I T8 T8 TE TE TE and TE respectively, of a bandpass filter consisting .of susceptance elements 21 and 21' each comprising three rods disposed in perpendicular relation to the wide plane of a rectangular waveguide 20. Let it be as sumed here that the long line of the cross-sectional rectangle of the rectangular waveguide lies in X-axis, the shorter line in Y-axis, and the longitudinal axis in Z-axis. Then, the electric field intensity distribution of the fundamental mode TE has a single sinusoidal hump each in X-Y plane and YZ plane (extending from the inductive rod 21 to 21). It should be noted here that the wavelength of the mode under consideration is the guide wavelength. FIG. 3(b) shows a field intensity distribution of higher mode TE having a double sinusoidal hump in X-Y plane, and a single hump in Y-Z plane (extending from the inductive rod 21 to 21'). FIG. 3(c) shows a similar distribution of higher mode TE having a single sinusoidal hump in the X-Y plane, and a double hump in the Y-Z plane (extending from the inductive post 21 to 21'). Similarly, FIGS. 3(d), (e) and (I) show field intensity distributions of higher modes TE TE and TE respectively. To generalize, reference characters m and n" of the notation TE,,,,,, denote the number of the humps of the electric field intensity distribution observed in the X-Y and Y-Z planes.

It is assumed here that the inner width measured in X direction of the rectangular waveguide 20 is a, and the axial length of the waveguide section or cavity defined by rod arrays 21 and 21' and measured in the Z-axis direction I. Then, the guide wavelength Ag of the electromagnetic wave propagated in the waveguide in the TE mode is expressed by The resonant frequency f at the cavity length is given, from Eqs. (1) and (2), by

, fi (is) rl When normalized by the use of cutofi frequency f, c/2a) and cutoff wavelength Ar 2a), Equation (3) is modified FIG. 4 shows characteristic curves, which are the results of calculation from Equation (5). In 1 FIG. 4, the abscissa represents f/fl, (the resonant frequency for TE mode normalized by the cutofi frequency f c/2a) for the fundamental mode TE and the ordinate represents l/Ac (the length l of the cavity normalized by the cutoff wavelength Ac(= 2a) of the fundamental mode TE Parameters for these curves are numbers m and n. The legend in the parenthesis along the abscissa is taken to represent the frequency of the passband of the filter and the second harmonic. Curve 31 shows the relationship between the resonant frequency f for the fundamental mode TE and the cavity length l normalized by the cutofi' frequency and cutoff wavelength for the TE mode itself. Curve 32 shows the relationship between the resonant frequency f of a higher mode T13 and the cavity length I normalized by the cutoff frequency j and cutoff wavelength Ac of the fundamental mode TE Similarly, curves 33 through 37 show relationships between the resonant frequency f for higher modes TE TE TE TE, and TE respectively, and the cavity length normalized by the cutofi frequency f, and cutoff wavelength M for the fundamental wave TE mode.

It will be apparent that the resonant modes having the same number n" of humps of the electric field intensity distribution observed in the Y-Z plane, for example, TE TE and T13 have similar wavelengths in a waveguide but not in free space.

Generally, a rectangular-waveguide-type bandpass filter is designed to operate with f/f value of the desirable (or fundamental) frequency of the passband in the range between 1.4 and 1.8. For example, in WRJ-4-type waveguide for 4,000 MHz band use, f/f for the fundamental frequency value is between 1.4 and 1.63. With WRJ-6 type waveguide for 6,000 MHz band use, f/f for the fundamental frequency is in the range between 1.58 and 1.71. It is therefore apparent that second harmonic must be rejected in the f/f value range between 2.8 and 3.3 or between 3.1 and 3.6. To attain this objective, the resonance curves for the higher modes should never fall within this range. In FIG. 4, the hatched area satisfies this condition. More precisely, the area where l/M value lies in the range between 0.2 and 0.3 is favorable. The reason for this is as follows: While the l/Ac values lie in the region between 0.3 and 0.6, curves 33, 35, and 36 respectively for TE TE and TE, modes are existent, this is not favorable to elimination of higher modes. Similarly, in the region above 0.6, the resonant frequency for the fundamental mode TE corresponding to the fundamental frequency becomes lower than the desirable resonant frequency f, and very difficult to raise. In the region below 0.2, the resonant frequency for the fundamental frequency is unnecessarily high, and also difficult to lower. Therefore, by way of selecting the cavity length l to fall within this area, the second harmonic component can be rejected. This raises, however, the value of f/f of the fundamental frequency to a value ranging from 1.9 to 2.7, as shown by the curve 31. To restore this to the range between 1.4 and 1.8, a capacitive element must be inserted into the cavity of FIG. 3(a) and thus to reduce the resonant frequency for the fundamental mode TE Briefly, in the conventional filter, the cavity length l is taken in the range between 0.4 and 0.5 in the He (or 1.4 to 1.8 inf/f Therefore, the higher modes appear within the frequency region above f where disturbance is caused and where the second harmonic 2f0 is included. In this invention, therefore, in order to prevent the resonance of the second harmonic 2f0, the length l is selected in the range from 0.2 to 0.3 in He, wherein the higher modes on the second harmonic 2fo do not exist and the resonant frequency for the desirable (or fundamental) frequency is reduced. In this case, the resonant frequencies of the higher modes TE- TE TE TE and TE must be arranged so as not to allow f/f to come in the range between 2.4 and 3.6, because this region belongs to the undesirable second higher harmonics, which could possibly be resonant to the higher modes. The method for attaining this is as follows:

As indicated by the curve 32 in FIG. 4, it is sufficient for the higher mode TE to reduce its resonant frequency or to keep it unchanged. As shown in FIG. 3(b), the position at which the field of T15 mode is minimum on the center line E-E' on the major plane of the rectangular waveguide 20. This means that the resonant frequency of T15 mode can be reduced by inserting a capacitive rod at a position except for on the line -15. In the T case, the cutoff frequency f normalized by the cutoff frequency f, for TE mode, f /f is equal to 2, which is well above the frequency region for TE mode ranging from 1.4 and L8. In other words, as for TE mode, the capacitive rod may be disposed anywhere. Also, since the resonant frequencies for higher modes TE TE TE are high enough, there is no problem in rejecting the second harmonic. Therefore, consideration must be taken only for the modes TE "[13 and TE In order to prevent the resonant frequencies for the three modes TE TE and TE from coming down to reach a certain specific band as a result of insertion of capacitive-rod into the cavity, this capacitive rod must be disposed as such a point at which the field of each the higher modes TE T5 and TE is minimum. Also, in order to lower the resonant frequency of the fundamental wave TE the capacitive rod must be disposed at such a point at which the field of the fundamental mode TE is maximum as the center position of the wide plane of the waveguide which forms the resonator with the susceptance elements. For the TE mode, the capacitive rod should be disposed on the center line A-A' of the cavity at which the field of the same mode is minimum as shown in FIG. 3(c). For the TE mode, the capacitive rods should be positioned on the trisectional lines B-B and C-C' on the major plane along the longitudinal axis at which the field of the mode is minimum as shown in FIG. 3(d). For the TE mode similarly, the element is on the center line A-A' of the cavity and also the center line E-E' on the major plane of the rectangular waveguide, at the two lines the field is minimum as shown in FIG. 3(e). The positions common to the conditions for these modes are D and D at which the center line A-A of the cavity are in crossed relation with the trisectional lines 8-8 and C-C as shown in FIG. 3(d). As described above, the TE mode has no problem in rejecting the second harmonic component. In order to reject the TE mode, it is necessary to make the arrangement of the bandpass filter symmetrical because the field intensity distribution on one side is in opposite phase with that on the other side with respect to the center line E-E on the major plane, and because a mode having opposite phase does not occur within a symmetrical waveguide.

Embodiments of the invention will be further described referring to FIGS. 5 and 6. In order to prevent the resonant frequencies for higher modes other than TE from falling in the region below the frequency twice as high as the resonant frequency for the fundamental wave, the length I of the cavity of the rectangular waveguide section is made equal to %Ag, in contrast to the corresponding length m of the conventional bandpass filter and adjustable capacitive elements (screws) 41 and 41 are installed at two points, respectively, at which the trisectional lines 42 and 42' on the major plane of the rectangular waveguide 40 intersect with the bisector 43 of the interval 1 between the induction rods 44 and 44'. As will be understood from FIGS. 3(c), (d) and (e), and the description thereof, these points correspond to the points where the field intensity is minimum with respect to the higher modes TE TE, and TE and the same is substantially maximum with respect to the fundamental mode TE By the use of the adjustable capacitive screws 41 and 41, a desired passband can be attained for the fundamental mode TE keeping the resonant frequencies for higher modes outside of the area wherein the second harmonic of the fundamental frequency exists. tAllso, the T5 mode is rejected due to the symmetry of the I ter.

FIG. 6 shows the attenuation vs. frequency characteristics of the second-harmonic-rejecting bandpass filter of FIG. 5. As is apparent from the characteristic curve, this filter is capable of rejecting the higher modes in the frequency region twice as high as the resonant frequencies for fundamental mode. More specifically, this filter rejects the higher modes in the region where f/f ranges from 2.8 to 3.6, while the f/f, value for the passband for fundamental wave ranges from 1.4 to 1.8. Thus, the second harmonic wave component is eliminated.

FIGS. 7 through 9 show modifications of the embodiment in FIG. 5. These modifications are based on the fact that, as described above, the higher mode TE may be left out of consideration if the second harmonic is to be rejected and, therefore, the structure of the bandpass filter need not be symmetrical.

FIG. 7 shows a modification of the embodiment in FIG. 5 wherein only one of the adjustable capacitive elements (screws) of FIG. 5 is used. FIG. 7 embodies only the adjustable capacitive element 41 disposed at the intersection of the trisection line 42 and the bisector 43 in the manner of FIG. 5, the adjustable capacitive 41' in FIG. 5 being omitted in FIG. 7. The essential dimensions in FIG. 7 being the same as corresponding dimensions in FIG. 5.

FIG. 8 shows another modification wherein one of the adjustable capacitive elements (screw) is installed at the trisectional in the opposing major surface of the waveguide. FIG. 8 embodies the adjustable capacitive element 41 disposed at the intersection of the trisectional line 42 and the bisector 43 in one waveguide wide side in the manner of FIG. 5 but the ad justable capacitive element 41 is disposed at the intersection of the trisectional line 42' and the bisector 43 in the wide side of the waveguide opposite to that embodying the adjustable capacitive element 41. The essential dimensions in FIG. 7 being the same as corresponding dimensions in FIG. 5.

FiG. 9 shows still another modification wherein two capacitive elements 41 and 41' are spaced on trisectional line 42 on opposite sides of bisector $3 in the vicinity of the position corresponding to that of the capacitive element 41 of FIG. 7. The essential dimensions in FIG. 9 being identical with corresponding dimensions in FIG. 5. Likewise, the arrangements of FIGS. 5 and 8 may be modified by replacing the single capacitive element with a plurality of capacitive elements disposed at around the positions as in FIG. 5 or 8.

FIG. 10 shows an arrangement wherein the capacitive elements 41 and 41' are oppositely installed on the opposite major planes of the waveguide. Accordingly, in FIG. 10, element 4-1 is installed at the intersection of trisectional line 42 and bisector 43 in the upper major plane of the waveguide while element 41' is installed at the intersection of the trisectional line 42 and bisector 43 in the lower major plane of the waveguide, whereby the elements 41 and 41' are oppositely disposed in opposite major planes of the waveguide. The essential dimensions in FIG. 10 are the same as corresponding dimensions in FIG. 5. The similar arrangement may be made in connection with FIGS. 5 and 7.

In the embodiment and modifications, single stage bandpass filters have been described. Generally, thehigher harmonicsrejecting bandpass filter consists of a plurality of stages. Needless to say, the invention can be applied to such multistage bandpass filters. The invention is applicable also to the A-wavelength-coupling-type and direct-coupling-type higherharrnonics rejecting bandpass filters.

The rectangular waveguide employed in the above embodiment and modifications may be replaced by acircular or elliptic waveguide. Also, the number of the susceptance rods employed in the embodiment to define each stage of the filter may not necessarily be three. It may be two, four or any other arbitrary number. Furthermore, these susceptance rods may be of window shape or any other shape.

In the embodiment, two adjustable screws are employed as the variable capacitive element at symmetrical points on the major plane or planes. However, the number of the screws may be chosen arbitrarily. The positions of the capacitive ele ments may not necessarily be symmetrical.

Also, it will be apparent to the engineers in this technical field that the principle of the present invention is applicable to rejection of higher-than-second higher harmonics.

While the invention has been shown schematically and described in detail with reference to particular embodiments and modifications, it will be clearly understood that the general principles of this invention may be applied to those skilled in the art to other structures of the microwave bandpass filter without departing from the spirit of the invention.

We claim:

1. A rectangular waveguide filter having a passband for transmitting electromagnetic waves including a fundamental frequency and attenuating a second harmonic or said fundamental frequency in a fundamental mode TE keeping resonant frequencies of modes higher than said mode TE outside of the region where the second harmonic of said fundamental frequency exists comprising:

a rectangular waveguide having narrow and wide opposite walls; two susceptance means spaced in mutually parallel relation interiorly of said waveguide in a direction lengthwise of said wide walls and disposed transversely to said narrow walls; each of said means consisting of a plurality of rods spaced in mutually parallel relation in a further direction perpendicular to said transverse direction; said rods and said waveguide forming a resonator cavity; said two means so spaced as to provide a distance of one-third waveguide wavelength between lengthwise axes of corresponding rods in said respective means for enabling said cavity to provide said filter passband to transmit said fundamental frequency band and to attenuate said second harmonic while preventing resonant frequencies of modes higher than said mode TE from coming into a frequency region including said second harmonic; and

adjustable capacitive means disposed in at least one of said widewalls and adjusted to project one end into the interior of said cavity to function as a capacitive means only for said fundamental mode thereby providing said filter cavity passband; said last-mentioned means having an axis disposed in a position which is located between planes including corresponding axes of said respective susceptance means at one-half of said one-third waveguide wavelength therebetween and which is further located from an inner surface of at least one of said narrow walls proximate to said capacitive means at one-third of an overall distance between inner surfaces of said nan row walls.

2. The waveguide filter according to claim 1 in which said capacitive means comprises two adjustable screws having some spaced in mutually parallel relation in said one wide wall in a plane perpendicular to said narrow and wide walls, each screw adjusted to project one end into said cavity; said two screw axes spaced in two positions which include said firstmentioned position and are located said one-half of said one third waveguide wavelength distance between said two planes including said corresponding axes of said susceptance means and which include each of said two screw axes further located said one-third distance from said inner surface of an adjacent narrow wall and said two screw axes having said one-third distance therebetween.

3. The waveguide filter according to claim 1 in which said capacitive means comprises an adjustable screw disposed in said one wide wall to project one end into said cavity and having an axis in said position which is located said one-half of said one-third waveguide wavelength distance between said two planes including said two susceptance means corresponding axes and which is further located said one-third distance from said inner surface of said adjacent one narrow wall.

4. The waveguide filter according to claim 1 in which said capacitive means comprises two adjustable screws of which one is disposed in said one wide wall and a second in the opposite wide wall; said one and second screws adjusted to project respective one ends thereof into said cavity and having axes in said position which is located said one-half of said onethird waveguide wavelength distance between said two planes including said susceptance means corresponding axes and which is further located said one-third distance from inner surfaces of said respective narrow walls.

5. The waveguide filter according to claim 1 in which said capacitive means comprises two adjustable screws having axes spaced in mutually parallel relation in said one wide wall in a plane perpendicular to said wide walls and parallel to said narrow walls; said two screws adjusted to project corresponding ends into said cavity and to dispose opposing peripheral portions approximately at said position which is located said onehalf of said one-third waveguide wavelength distance between said two planes including said susceptance means corresponding axes and which is further located said one-third distance from said inner surface of said adjacent one narrow wall.

6. The waveguide filter according to claim 1 in which said filter means comprises two adjustable screws of which one is disposed in said one wide wall and a second in the opposite wide wall; said one and second screws adjusted to project respective one ends into said cavity and having axes in said position which is located said one-half of said one-third waveguide wavelength distance between said two planes including said two susceptance means corresponding axes and which is further located said one-third distance from said inner surface of said one narrow wall.

7. A rectangular waveguide filer having a passband for transmitting a band of electromagnetic waves including a fundamental frequency and attenuating at least a second harmonic of said fundamental frequency in a fundamental mode T5 comprising:

a rectangular waveguide having narrow and wide opposite walls; two susceptance means spaced in mutually parallel relation interiorly of said waveguide in a direction lengthwise of said wide walls and disposed transversely to said narrow walls; each of said means consisting of a plurality of rods spaced in mutually parallel relation in a further direction perpendicular to said transverse direction; said waveguide and said rods forming a resonator cavity for providing said passband to transmit said waves; said two means so spaced as to provide a distance of one-third waveguide wavelength between lengthwise axes of corresponding rods in said respective means for enabling said cavity to transmit said fundamental band and to'attenuate said second harmonic while preventing resonant frequencies of modes higher than said modefrom coming into a frequency region including said second harmonic; and

two adjustable screws having lengthwise axes spaced in mutually parallel relation in one wide wall in a first plane perpendicular to said narrow and wide walls; said two screws adjusted to project corresponding ends into said cavity to function as capacities only for said fundamental mode to provide said filter passband; said first plane equidistantly located between two other planes including said lengthwise axes of said rods of said respective susceptance means at a distance of one-half of said onethird waveguide wavelength distance; said screw lengthwise axes further located from inner surfaces of respective adjacent narrow walls at one-third of an overall distance between said last-mentioned surfaces and said screw lengthwise axes having said one-third distance therebetween in said first plane.

8. A rectangular waveguide filter having a passband for transmitting a band of electromagnetic waves including a fundamental frequency and attenuating at least a second harmonic of said fundamental frequency in a fundamental mode TE comprising:

a rectangular waveguide having narrow and wide opposite walls;

two susceptance means spaced in mutually parallel relation interiorly of said waveguide in a direction lengthwise of said wide walls and disposed transversely to said narrow walls; each of said means consisting of a plurality of rods spaced in mutually parallel relation in a further direction perpendicular to said transverse direction; said waveguide and said rods forming a resonator cavity for transmitting said waves; said two means so spaced as to provide a distance of one-third waveguide wavelength between lengthwise axes of corresponding rods in said respective means for enabling said cavity to transmit said fundamental band and to attenuate said second harmonic while preventing resonant frequencies of modes higher than said fundamental mode from coming into a frequency region including said second harmonic; and

an adjustable screw disposed in one wide wall and adjusted to project one end into said cavity to function as a capacity only for said fundamental mode to provide said filter passbands; said screw having a lengthwise axis equidistantly located between two planes including lengthwise axes of said rods of said respective susceptance means at a distance of one-half of said onethird waveguide wavelength distance; said screw lengthwise axis further located from an inner surface of an adjacent narrow wall at one-third of an overall distance between inner surfaces of said narrow walls.

9. A rectangular waveguide filter having a passband for transmitting a band of electromagnetic waves including a fundamental frequency and attenuating at least a second harmonic of said fundamental frequency in a fundamental mode TE comprising:

a rectangular waveguide having narrow and wide opposite walls;

two susceptance means spaced in mutually parallel relation interiorly of said waveguide in a direction lengthwise of said wide walls and disposed transversely to said narrow walls; each of said means consisting of a plurality of rods spaced in mutually parallel relation in a further direction perpendicular to said transverse direction; said waveguide and said rods forming a resonator cavity for transmitting said waves; said two means so spaced as to provide a distance of one-third waveguide wavelength between lengthwise axes of corresponding rods in said respective two means for enabling said cavity to transmit said fundamental band and to attenuate said second harmonic while preventing resonant frequencies of modes higher than said fundamental mode from coming into a frequency region including said second harmonic; and two adjustable screws of which one is disposed in one wide wall and a second in the opposite wide wall; said one and second screws adjusted to project respective one ends thereof into said cavity to function as capacities only for said fundamental mode to provide said filter passband; said two screws having lengthwise axes equidistantly located between two planes including lengthwise axes of said rods of said respective two means at a distance of one-half of said one-third waveguide wavelength distance; said one and second screw lengthwise axes further located from inner surfaces of said respective adjacent narrow walls at one-third of an overall distance between said last-mentioned inner surfaces.

10. A rectangular waveguide filter having a passband for transmitting a band of electromagnetic waves including a fundamental frequency and attenuating at least a second harmonic of said fundamental frequency in a fundamental mode TE comprising:

a rectangular waveguide having narrow and wide opposite walls;

two susceptance means spaced in mutually parallel relation interiorly of said waveguide in a direction lengthwise of said wide walls and disposed transversely to said narrow walls; each of said means consisting of a plurality of rods spaced in mutually parallel relation in a further direction perpendicular to said transverse direction; said waveguide and rods forming a resonator cavity for trans mitting said waves; said two means so spaced as to provide a distance of one-third waveguide wavelength between lengthwise axes of corresponding rods in said respective two means for enabling said cavity to transmit said fundamental band and to attenuate said second harmonic while preventing resonant frequencies of modes higher than said fundamental mode from coming into a frequency region including said second harmonic; and two adjustable screws spaced in mutually parallel relation in one wide wall and adjusted to project respective one ends thereof into said cavity to function as capacities only for said fundamental mode to provide said filter passband; said two screws having lengthwise axes disposed in parallel in a plane perpendicular to said wide walls and parallel to said narrow walls and also having opposing peripheral waveguide and rods forming a resonating cavity for transmitting said waves; said two means so spaced as to provide a distance of one-third waveguide wavelength between lengthwise axes of corresponding rods in said portions located approximately at a distance equidistantly respective two means for enabling; said cavity to transmit between two planes including lengthwise axes of said rods said fundamental band and to attenuate said second harof said respective susceptance means at one-half of said monic while preventing resonant frequencies of modes one-third waveguide wavelength distance; said two gh rh n Said a ental mode from Com g nto 3 screws having said lengthwise axes thereof located from frequency region including said second harm and an inner surface of an adjacent narrow wall at one-third 10 two adjustable SCIEWS Of which O e is d spmed in One wide of an overall distance between said narrow walls. Wall and a second in the pp i Wide Wall; S i one and 11. A rectangular waveguide filter having a passband for Second SCYFWS JQ l' 'Ql f pq s p ftransmitting a band of electromagnetic waves including a fun- W WW- eflds Sald cavlty t0 as F p damental frequency and attenuating at least a second hartles y sijud fundamental to Provide 9 filter monic of said fundamental frequency, comprising: p s a 531d CW0 Screws having lengthwlse alias a rectangular waveguide having narrow and wide opposite q f y ed between IW p i Including walls; lengthwise axes of said rods in sa d respect ve two means two susceptance means spaced in mutually parallel relation at a dlstanceof of Said one'thlrd wavegulde interiorly of said waveguide in a direction lengthwise of Wavelength dlstance; two screw lengthwlse axes said wide walls and disposed transversely to said narrow further loFated from surface of one naFmw wall walls; each of said means consisting of a plurality of rods at one'thlrd of an overall dlstance between Said narrow spaced in mutually parallel relation in a further direction wallsperpendicular to said transverse direction; said 

1. A rectangular waveguide filter having a passband for transmittinG electromagnetic waves including a fundamental frequency and attenuating a second harmonic or said fundamental frequency in a fundamental mode TE101 keeping resonant frequencies of modes higher than said mode TE101 outside of the region where the second harmonic of said fundamental frequency exists comprising: a rectangular waveguide having narrow and wide opposite walls; two susceptance means spaced in mutually parallel relation interiorly of said waveguide in a direction lengthwise of said wide walls and disposed transversely to said narrow walls; each of said means consisting of a plurality of rods spaced in mutually parallel relation in a further direction perpendicular to said transverse direction; said rods and said waveguide forming a resonator cavity; said two means so spaced as to provide a distance of one-third waveguide wavelength between lengthwise axes of corresponding rods in said respective means for enabling said cavity to provide said filter passband to transmit said fundamental frequency band and to attenuate said second harmonic while preventing resonant frequencies of modes higher than said mode TE101 from coming into a frequency region including said second harmonic; and adjustable capacitive means disposed in at least one of said wide walls and adjusted to project one end into the interior of said cavity to function as a capacitive means only for said fundamental mode thereby providing said filter cavity passband; said last-mentioned means having an axis disposed in a position which is located between planes including corresponding axes of said respective susceptance means at one-half of said one-third waveguide wavelength therebetween and which is further located from an inner surface of at least one of said narrow walls proximate to said capacitive means at one-third of an overall distance between inner surfaces of said narrow walls.
 2. The waveguide filter according to claim 1 in which said capacitive means comprises two adjustable screws having some spaced in mutually parallel relation in said one wide wall in a plane perpendicular to said narrow and wide walls, each screw adjusted to project one end into said cavity; said two screw axes spaced in two positions which include said first-mentioned position and are located said one-half of said one-third waveguide wavelength distance between said two planes including said corresponding axes of said susceptance means and which include each of said two screw axes further located said one-third distance from said inner surface of an adjacent narrow wall and said two screw axes having said one-third distance therebetween.
 3. The waveguide filter according to claim 1 in which said capacitive means comprises an adjustable screw disposed in said one wide wall to project one end into said cavity and having an axis in said position which is located said one-half of said one-third waveguide wavelength distance between said two planes including said two susceptance means corresponding axes and which is further located said one-third distance from said inner surface of said adjacent one narrow wall.
 4. The waveguide filter according to claim 1 in which said capacitive means comprises two adjustable screws of which one is disposed in said one wide wall and a second in the opposite wide wall; said one and second screws adjusted to project respective one ends thereof into said cavity and having axes in said position which is located said one-half of said one-third waveguide wavelength distance between said two planes including said susceptance means corresponding axes and which is further located said one-third distance from inner surfaces of said respective narrow walls.
 5. The waveguide filter according to claim 1 in which said capacitive means comprises two adjustable screws having axes spaced in mutually parallel relation in said one wide wall in a plane perpendicular to said wide walls and parallel to said narrow walls; said two screws adjusted to project coRresponding ends into said cavity and to dispose opposing peripheral portions approximately at said position which is located said one-half of said one-third waveguide wavelength distance between said two planes including said susceptance means corresponding axes and which is further located said one-third distance from said inner surface of said adjacent one narrow wall.
 6. The waveguide filter according to claim 1 in which said filter means comprises two adjustable screws of which one is disposed in said one wide wall and a second in the opposite wide wall; said one and second screws adjusted to project respective one ends into said cavity and having axes in said position which is located said one-half of said one-third waveguide wavelength distance between said two planes including said two susceptance means corresponding axes and which is further located said one-third distance from said inner surface of said one narrow wall.
 7. A rectangular waveguide filter having a passband for transmitting a band of electromagnetic waves including a fundamental frequency and attenuating at least a second harmonic of said fundamental frequency in a fundamental mode TE101, comprising: a rectangular waveguide having narrow and wide opposite walls; two susceptance means spaced in mutually parallel relation interiorly of said waveguide in a direction lengthwise of said wide walls and disposed transversely to said narrow walls; each of said means consisting of a plurality of rods spaced in mutually parallel relation in a further direction perpendicular to said transverse direction; said waveguide and said rods forming a resonator cavity for providing said passband to transmit said waves; said two means so spaced as to provide a distance of one-third waveguide wavelength between lengthwise axes of corresponding rods in said respective means for enabling said cavity to transmit said fundamental band and to attenuate said second harmonic while preventing resonant frequencies of modes higher than said mode from coming into a frequency region including said second harmonic; and two adjustable screws having lengthwise axes spaced in mutually parallel relation in one wide wall in a first plane perpendicular to said narrow and wide walls; said two screws adjusted to project corresponding ends into said cavity to function as capacities only for said fundamental mode to provide said filter passband; said first plane equidistantly located between two other planes including said lengthwise axes of said rods of said respective susceptance means at a distance of one-half of said one-third waveguide wavelength distance; said screw lengthwise axes further located from inner surfaces of respective adjacent narrow walls at one-third of an overall distance between said last-mentioned surfaces and said screw lengthwise axes having said one-third distance therebetween in said first plane.
 8. A rectangular waveguide filter having a passband for transmitting a band of electromagnetic waves including a fundamental frequency and attenuating at least a second harmonic of said fundamental frequency in a fundamental mode TE101, comprising: a rectangular waveguide having narrow and wide opposite walls; two susceptance means spaced in mutually parallel relation interiorly of said waveguide in a direction lengthwise of said wide walls and disposed transversely to said narrow walls; each of said means consisting of a plurality of rods spaced in mutually parallel relation in a further direction perpendicular to said transverse direction; said waveguide and said rods forming a resonator cavity for transmitting said waves; said two means so spaced as to provide a distance of one-third waveguide wavelength between lengthwise axes of corresponding rods in said respective means for enabling said cavity to transmit said fundamental band and to attenuate said second harmonic while preventing resonant frequencies of modes higher than said fundamental mode from coming into a frequency rEgion including said second harmonic; and an adjustable screw disposed in one wide wall and adjusted to project one end into said cavity to function as a capacity only for said fundamental mode to provide said filter passbands; said screw having a lengthwise axis equidistantly located between two planes including lengthwise axes of said rods of said respective susceptance means at a distance of one-half of said one-third waveguide wavelength distance; said screw lengthwise axis further located from an inner surface of an adjacent narrow wall at one-third of an overall distance between inner surfaces of said narrow walls.
 9. A rectangular waveguide filter having a passband for transmitting a band of electromagnetic waves including a fundamental frequency and attenuating at least a second harmonic of said fundamental frequency in a fundamental mode TE101, comprising: a rectangular waveguide having narrow and wide opposite walls; two susceptance means spaced in mutually parallel relation interiorly of said waveguide in a direction lengthwise of said wide walls and disposed transversely to said narrow walls; each of said means consisting of a plurality of rods spaced in mutually parallel relation in a further direction perpendicular to said transverse direction; said waveguide and said rods forming a resonator cavity for transmitting said waves; said two means so spaced as to provide a distance of one-third waveguide wavelength between lengthwise axes of corresponding rods in said respective two means for enabling said cavity to transmit said fundamental band and to attenuate said second harmonic while preventing resonant frequencies of modes higher than said fundamental mode from coming into a frequency region including said second harmonic; and two adjustable screws of which one is disposed in one wide wall and a second in the opposite wide wall; said one and second screws adjusted to project respective one ends thereof into said cavity to function as capacities only for said fundamental mode to provide said filter passband; said two screws having lengthwise axes equidistantly located between two planes including lengthwise axes of said rods of said respective two means at a distance of one-half of said one-third waveguide wavelength distance; said one and second screw lengthwise axes further located from inner surfaces of said respective adjacent narrow walls at one-third of an overall distance between said last-mentioned inner surfaces.
 10. A rectangular waveguide filter having a passband for transmitting a band of electromagnetic waves including a fundamental frequency and attenuating at least a second harmonic of said fundamental frequency in a fundamental mode TE101, comprising: a rectangular waveguide having narrow and wide opposite walls; two susceptance means spaced in mutually parallel relation interiorly of said waveguide in a direction lengthwise of said wide walls and disposed transversely to said narrow walls; each of said means consisting of a plurality of rods spaced in mutually parallel relation in a further direction perpendicular to said transverse direction; said waveguide and rods forming a resonator cavity for transmitting said waves; said two means so spaced as to provide a distance of one-third waveguide wavelength between lengthwise axes of corresponding rods in said respective two means for enabling said cavity to transmit said fundamental band and to attenuate said second harmonic while preventing resonant frequencies of modes higher than said fundamental mode from coming into a frequency region including said second harmonic; and two adjustable screws spaced in mutually parallel relation in one wide wall and adjusted to project respective one ends thereof into said cavity to function as capacities only for said fundamental mode to provide said filter passband; said two screws having lengthwise axes disposed in parallel in a plane perpendicular to said wide walls and parallel to said narrow walls and also having opposing peripheral portions located approximately at a distance equidistantly between two planes including lengthwise axes of said rods of said respective susceptance means at one-half of said one-third waveguide wavelength distance; said two screws having said lengthwise axes thereof located from an inner surface of an adjacent narrow wall at one-third of an overall distance between said narrow walls.
 11. A rectangular waveguide filter having a passband for transmitting a band of electromagnetic waves including a fundamental frequency and attenuating at least a second harmonic of said fundamental frequency, comprising: a rectangular waveguide having narrow and wide opposite walls; two susceptance means spaced in mutually parallel relation interiorly of said waveguide in a direction lengthwise of said wide walls and disposed transversely to said narrow walls; each of said means consisting of a plurality of rods spaced in mutually parallel relation in a further direction perpendicular to said transverse direction; said waveguide and rods forming a resonating cavity for transmitting said waves; said two means so spaced as to provide a distance of one-third waveguide wavelength between lengthwise axes of corresponding rods in said respective two means for enabling said cavity to transmit said fundamental band and to attenuate said second harmonic while preventing resonant frequencies of modes higher than said fundamental mode from coming into a frequency region including said second harmonic; and two adjustable screws of which one is disposed in one wide wall and a second in the opposite wide wall; said one and second screws adjusted to project corresponding respective opposite ends into said cavity to function as capacities only for said fundamental mode to provide said filter passband; said two screws having lengthwise axes equidistantly located between two planes including lengthwise axes of said rods in said respective two means at a distance of one-half of said one-third waveguide wavelength distance; said two screw lengthwise axes further located from an inner surface of one narrow wall at one-third of an overall distance between said narrow walls. 